Introduction

Intellectual disability (ID), also known as learning disability, is a neurodevelopmental disorder characterised by significant deficits in intellectual and adaptive behaviour functioning that begin in the developmental period [1]. A large proportion of adults with ID are disadvantaged by disproportionately high rate of physical and mental health comorbidities compared to those without ID. Due to a lack of research in this patient group, the evidence base on diagnosis and management of physical and mental health conditions continues to lag behind in adults with ID. Non-accessible research designs, lack of confidence with the intellectual capacity of individuals ability to consent, limited resources such as time, and a need for training have been cited as barriers for this reduced evidence base [2]. It has also been reported that ethics committees can be reluctant to include adults with ID in research in order to ‘protect’ them. This has further disadvantaged adults with ID, who are already affected by high morbidity and mortality.

Sleep medicine is amongst the many areas that lacks a robust evidence base in adults with ID. Exclusion of adults with ID from most research in sleep has led to a limited evidence base in this population. We argue focusing on sleep disorders in adults with ID is an important step towards reducing inequality and widening participation. This paper aims to contribute to an overdue paradigm shift in awareness and action to redress this prejudice and neglect in research.

Current Evidence Base on Sleep and Sleep Disorders in People with ID

Despite a recent significant expansion of our understanding of sleep and sleep disorders, our knowledge of how these advancements pertain to adults with ID remains limited [3]. Sleep disorders in adults with ID are often referred in the literature under different terminologies, such as behavioural sleep problems, which may encompass any sleep disorder from insomnia to narcolepsy [4]. Despite internationally recognised diagnostic categories and criteria, it reflects the limited advancement of sleep medicine in this population. Diagnostic overshadowing, where clinicians may attribute a sleep disorder to being part of a person’s intellectual disabilities, rather than recognising and treating it as a separate entity [5] further affect appropriate support for individuals experiencing sleep problems. Therefore, diagnostic methods and treatments of sleep disorders in adults with ID is extrapolated from the evidence base in adults without ID, except for specific studies looking at certain sleep disorders in specific genetic conditions.

Below, we summarise the current understanding of common sleep disorders in adults with ID.

Insomnia

Insomnia is more common in adults with ID when compared to the general population [6]. One of the reasons underlying this, may be that adults with ID experience more fragmented sleep [7]. There is also growing evidence that risk of sleep disturbance increases with the severity of the ID [8, 9]. It is likely that sleep problems are only reported when they become a challenge to caregivers, especially in those individuals who cannot communicate their sleep-related night- and day- time dysfunction. There are no specific guidelines for diagnosing Chronic Insomnia Disorder in this cohort.

Cognitive Behavioural Therapy for Insomnia (CBT-I) is the first line treatment recommended by UK national [10], as well as international sleep medicine guidelines [11]. There is some evidence that a modified CBT-I approach is helpful for adults with ID [12]. Focusing on sleep scheduling has been successfully demonstrated, either on its own or as part of a multi-modal CBT-I approach for adults with ID. In this technique, the average total sleep time (obtained from sleep diaries and/or actigraphy) is aligned to the average total time in bed, usually measured over 1–2 weeks. This has been shown to optimise the homeostatic sleep drive, thereby consolidating sleep, and reducing hyperarousal. This technique requires caregiver education and flexibility, as often adults with ID have early bedtimes imposed on them i.e. they are ‘put to bed’ before their homeostatic sleep drive is sufficiently high enough to induce and/or maintain sleep. Sleep scheduling should be supervised by a behavioural sleep medicine specialist, as any unintended sleep loss may worsen physical (e.g. epilepsy) and mental (e.g. bipolar affective) disorders. Moreover, if a sleep diary is completed by a caregiver (as opposed to the individual with ID), there may be reporting errors, such as a lower estimate of wake-after-sleep-onset time (a measure of sleep maintenance) [13]. Additional CBT-I techniques are outlined in Table 1.

Table 1 Modified Cognitive Behavioural Therapy for Insomnia (CBT-I) for Individuals with Intellectual Disabilities. From reference [31]
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Optimising scheduled daytime activities and sleeping environments can also significantly improve sleep in this population. For example, it has been reported that adults with ID attain less daily exercise, have relatively unhealthy diets, and reduced access to structured daytime activities when compared to the general population [12].

The pharmacological treatment of Chronic Insomnia Disorder in adults with ID tends to follow the same treatment guidelines as those for the general population. There is again a paucity of research in this area, with melatonin receiving the most attention on account of its favourable side-effect profile. One meta-analysis concluded that in individuals with intellectual disabilities, the use of melatonin decreases sleep onset latency (i.e. the time taken to get to sleep), decreases the number of awakenings per night, and increases the total sleep time [14]. Despite the growing evidence base for pharmacological treatments, as far as we are aware, none of these studies include people with ID.

Sleep Breathing Disorders – Obstructive Sleep Apnoea

Little is known about the prevalence of sleep breathing disorders in adults with ID, with one notable exception, i.e. Obstructive Sleep Apnoea (OSA) in adults with Down syndrome (DS) [15], where prevalence rates of up to 100% have been reported [16]. Adults with DS are at increased risk of OSA because of the characteristic features such as cranio-facial abnormalities, hypotonia, and obesity associated with the genetic syndrome. As a result, it is recommended that everyone with DS in the UK is screened for OSA [17].

OSA in DS presents similarly to the general population, with symptoms of snoring, witnessed apnoea, nocturia, and excessive daytime somnolence as well as more atypical symptoms, including behavioural and mood disturbances [18]. This is important in the context of interventions for challenging behaviour in people with ID. Furthermore, there is evidence that optimal brain ageing is important for adults with DS, and reducing nocturnal hypoxia is important, given their increased risk of early onset Alzheimer’s dementia [19].

As with other sleep disorders, OSA symptom reports in adults with DS often rely on caregiver collateral history and observation, and it can be further complicated by diagnostic overshadowing. To aid clinician screening, the STOP-Bang questionnaire has been validated for adults with DS, who have a moderate-to-severe OSA [20]; and there is also a pictorial version of the Epworth Sleepiness Scale, which has had more variable validation success [21]. The STOP-Bang questionnaire consists of four self-reportable (STOP: snoring, tiredness, observed apnoea, and high blood pressure) and four demographic (Bang: body mass index, age, neck circumference, and gender) items [22]. Individuals with a STOP-Bang score of 0 to 2 can be classified as low risk for moderate to severe OSA, whereas those with a score of 5 to 8 can be classified as high risk for moderate to severe OSA. Individuals whose STOP-Bang scores are in the midrange (i.e. 3 or 4), require further clinical evaluation to determine the likelihood of OSA [22].

To aid diagnosis, it is generally advocated to use the least intrusive sleep diagnostic investigation, preferably undertaken in a home setting, e.g. with home pulse oximetry. If inpatient polysomnography is clinically indicated, then having access to an individual room, as well as the capacity to involve caregivers can be helpful, as can allowing extra time for equipment (e.g. electrode) acclimatation. The world of sleep diagnostics is a rapidly evolving one, and the expansion of minimally invasive and contactless sleep recording [23] hold great promise for improving SDB diagnoses in adults with ID. There may of course be times when an adult with ID cannot tolerate any clinically relevant sleep investigations, and in this scenario, a pragmatic trial of treatment may be required.

Continuous positive airway pressure (CPAP) has been demonstrated to be effective in treating adults with DS and OSA, with improvements in sleep, excessive daytime somnolence, mood, behavioural disturbance, general health and cognitive function [24]. CPAP adherence is always a challenge (regardless of the presence of ID), and encouragingly, one observational study of 75 adults with DS and OSA, demonstrated a mean CPAP concordance of > 4 h in 79% of participants [25]. A CPAP concordance of > 4 h/night is the general minimum advised treatment time in the non-ID population to attain sleep and health benefits [26]. However, using this same recommendation in the ID population may not be warranted e.g. another study of 25 adults with DS and OSA demonstrated that 2.8 h CPAP use/night over a 12-month period led to appreciable health, mood and behavioural benefits [24]. For individuals who struggle to accept CPAP, exposure therapy is considered beneficial, and in this scenario, collaborative working between sleep medicine specialists and others (e.g., mental health nurses, families, carers) can be helpful. Where positive airway pressure is unsuccessful or not tolerated, there is some evidence to support sleep surgery, such as hypoglossal nerve stimulation [27]; and where obesity plays an important aetiological role, medical management e.g. with glucagon receptor-1 agonists, can be considered [28]. Interestingly, a 10-year follow-up study of individuals with DS and OSA who underwent sleep surgery (e.g. hypoglossal nerve stimulation, adenotonsillectomy) demonstrated reduced morbidity and mortality compared to those who had not [29].

Other Sleep Disorders

At least a third of adults with ID will experience multiple sleep difficulties [30], with older adults, and those with a higher severity of ID and/or associated physical and mental health co-morbidities at higher risk [31].

Circadian Rhythm Sleep Wake Disorders, and Sleep Movement Disorders, such as restless legs syndrome and periodic limb movements in sleep are other sleep disorders of importance in people with ID given their association with Neurodevelopmental disorders (NDDs) [32], such as Autism Spectrum Disorder (ASD) and Attention Deficit Hyperactivity Disorder (ADHD).

Other factors such as common physical health conditions, including constipation, pain, wisdom teeth eruption, and gastroesophageal reflux disease can often go unrecognised, particularly in non-verbal adults with ID, resulting in quite significant sleep disruption [31]. Certain genetic and syndromic ID disorders are also associated with specific sleep disorders e.g. circadian rhythm sleep–wake cycle inversion in Smith-Magenis Syndrome [33], hypersomnolence/narcolepsy in Prader-Willi Syndrome [34], nocturnal seizures in Angelman Syndrome [35], and early-onset dementia with sleep disturbance in Down syndrome [36]. Moreover, common mental health co-morbidities such as depression and anxiety that may present atypically in adults with ID [37], and there may be increased sensitivities to medications [38], all of which may present as a primary sleep disturbance.

There are no published sleep medicine guidelines for adults with ID on specific assessment nor treatment. We have previously proposed one approach for the screening, assessment and treatment of sleep disorders in adults with ID (Table 2 and Fig. 1) [3, 31]. In our experience, treatment plans frequently require a holistic multi-modal approach, which may in part be based on existing standard adult sleep medicine treatment guidelines.

Table 2 Sleep Assessment – a 7-step approach guide. From reference: [31]
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Fig. 1
figure 1

Suggested flowchart for screening, assessment, and diagnosis of sleep disorders in adults with intellectual disability. Abbreviations: ID intellectual disability, CBT-I cognitive behavioural therapy for insomnia, GP general practitioner, MDT multiple disciplinary team. From reference [31]

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Areas That Need Further Research

We suggest four broad areas for research on sleep disorders in adults with ID, as a foundation to develop an evidence base.

  1. 1.

    It is important to understand how sleep disorders manifest in adults with ID, and as highlighted above, not assume that they present in the same way as the general population. This is of particular importance where sleep diagnoses are dependent upon subjective symptom reporting. As an analogy, successful adaptation of mental health diagnostic criteria (e.g. Diagnostic Manual – Intellectual Disability for mental disorders in people with ID) has helped to improve diagnostic accuracy, treatment, and quality of life for adults with ID.

  2. 2.

    Well-designed epidemiological studies are required to understand the prevalence and determinants of sleep disorders in adults with ID. Several factors such as higher rates of physical, mental and neurodevelopmental comorbidities can further affect the prevalence rate of sleep disorders in people with ID. For example, a sleep disorder in a person with ID and ADHD, or a person with ID and epilepsy, is likely to be different from someone with ID without comorbidities.

  3. 3.

    Improving access to sleep investigations, which previously may not have been tolerated by adults with ID. There is every reason to be hopeful in this area, particularly with the expansion of non-contactable sleep investigation devices.

  4. 4.

    Finally, evaluation of existing sleep disorder treatments in the general population for their usage and validity in people with ID is important. For many reasons, people with ID are different from one to another. For example, the aetiology of the ID can vary from: a genetic condition to a metabolic disorder, or a brain injury, or to an unknown aetiology. Including all of these people under one category will invariably affect any research findings, as they may have different: sleep disorder pathologies, mental and physical co-morbidities, and treatment responses due to their differing pharmacokinetics and pharmacodynamics. Similar research strategies have helped advance our understanding of the presentation, prevalence, investigation and treatment of OSA in adults with DS.

Conclusion

Sleep disorders are very common in adults with ID. They adversely affect mental and physical health, as well as impair quality of life for the adult with ID, as well as their caregivers. Despite this, our current understanding of sleep disorder prevalence, manifestation, and therapeutic interventions in its infancy. Future research in each of these areas is required in order to improve the diagnosis and treatment of sleep disorders in adults with ID. This will help to close the sleep medicine inequality gap in this often-overlooked group.

Key Points

  1. 1.

    Adults with ID are a heterogenous group of people with frequent comorbid mental, physical and neurodevelopmental disorders.

  2. 2.

    The sleep disorder evidence base in adults with ID is limited.

  3. 3.

    There is a paucity of evidence to support the transposition of general adult assessment, diagnostic and treatment sleep disorder guidelines onto adults with ID.

  4. 4.

    Future research should be aimed at reducing this divide, and bring sleep equity to adults with ID.